Semiconductor structure

ABSTRACT

A semiconductor structure includes a substrate, a channel layer, a barrier layer, a source structure, a drain structure, a doped compound semiconductor layer, a dielectric layer, and a gate structure. The channel layer is disposed on the substrate. The barrier layer is disposed on the channel layer. The source structure and the drain structure are disposed on opposite sides of the barrier layer. The doped compound semiconductor layer is disposed on the barrier layer. The doped compound semiconductor layer has a first side adjacent to the source structure and a second side adjacent to the drain structure. The doped compound semiconductor layer has at least one opening exposing at least a portion of the barrier layer. The dielectric layer is disposed on the doped compound semiconductor layer and the barrier layer. The gate structure is disposed on the doped compound semiconductor layer.

BACKGROUND Field of the Disclosure

The present disclosure relates to a semiconductor structure, and inparticular, to a semiconductor structure with a doped compoundsemiconductor layer.

Description of the Related Art

Gallium nitride-based (GaN-based) semiconductor materials have manyexcellent characteristics, for example, a high thermal resistance, awide band-gap, and a high electron saturation rate. Therefore, GaN-basedsemiconductor materials can be applied to high-speed andhigh-temperature operating environments. In recent years, GaN-basedsemiconductor materials have been widely used in light-emitting diode(LED) elements and high-frequency elements, such as high electronmobility transistors (HEMT) with structures of heterogeneous interfaces.

During manufacturing process of a HEMT, semiconductor materials of theHEMT may be adversely affected by the environment, such as thetemperature or elements in the environment, resulting in deactivation ofthe semiconductor materials and lower gate controllability of thedevice. This may further degrade the capability to drive current andimpact the electrical uniformity of products in different batchesfabricated in the same or similar processes.

With developments of GaN-based semiconductor materials, thosesemiconductor devices with GaN-based semiconductor materials are used inharsh working conditions, such as higher frequencies, highertemperatures or higher voltages. Therefore, semiconductor devices withGaN-based semiconductor materials still need to be improved to overcomethe challenges.

SUMMARY

An embodiment of the present disclosure provides a semiconductorstructure, which includes a substrate, a channel layer, a barrier layer,a source structure, a drain structure, a doped compound semiconductorlayer, a dielectric layer, and a gate structure. The channel layer isdisposed on the substrate. The barrier layer is disposed on the channellayer. The source structure and the drain structure are disposed onopposite sides of the barrier layer. The doped compound semiconductorlayer is disposed on the barrier layer. The doped compound semiconductorlayer has a first side adjacent to the source structure and a secondside adjacent to the drain structure. The doped compound semiconductorlayer has at least one opening exposing at least a portion of thebarrier layer. The dielectric layer is disposed on the doped compoundsemiconductor layer and the barrier layer. The gate structure isdisposed on the doped compound semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the disclosure can be more fully understood byreading the subsequent detailed description and examples with referencesmade to the accompanying drawings. In accordance with the standardpractice in the industry, various features are not drawn to scale. Infact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 illustrates a top view of a portion of a semiconductor structure,in accordance with some embodiments of the present disclosure.

FIG. 2 illustrates a cross-sectional view of the semiconductor structurealong the A-A′ line in FIG. 1 .

FIG. 3 illustrates a top view of a portion of a semiconductor structure,in accordance with some embodiments of the present disclosure.

FIG. 4 illustrates a cross-sectional view of the semiconductor structurealong the A-A′ line in FIG. 3 .

FIGS. 5-11 illustrate top views of portions of semiconductor structures,in accordance with other embodiments of the present disclosure.

FIG. 12 illustrates intensities of electric fields near surfaces ofsemiconductor structures.

DETAILED DESCRIPTION OF THE DISCLOSURE

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Some embodiments of the disclosure are described below. Additionaloperations can be provided before, during, and/or after the stagesdescribed in these embodiments. Some of the stages that are describedcan be replaced or eliminated for different embodiments. Additionalfeatures can be added to the semiconductor structure. Some of thefeatures described below can be replaced or eliminated for differentembodiments. Although some embodiments are discussed with operationsperformed in a particular order, these operations may be performed inanother logical order. Furthermore, the term “about” means a given valuemay vary with a specific technology node related to the desiredsemiconductor device. In some embodiments, based on specific technologynodes, the term “about” may represent, for example, a given value withina range between 10% and 30% of that value, such as within ±10%, ±20%, or±30% of that value.

The embodiments of the present disclosure provide a semiconductorstructure with reduced proportion of the doped compound semiconductorlayer in the semiconductor structure, which may improve electricaluniformity and performance of the device. In some embodiments, deviceperformance may be improved through the liner layer or the protectionlayer disposed on sidewalls of the doped compound semiconductor layerand the barrier layer, or through the liner layer disposed under thesource electrode and the drain electrode.

FIG. 1 illustrates a top view of a portion of a semiconductor structureand FIG. 2 illustrates a cross-sectional view of the semiconductorstructure along the A-A′ line in FIG. 1 , in accordance with someembodiments of the present disclosure. The semiconductor structure 200includes the substrate 110, the channel layer 112, the barrier layer113, the source structure containing the source electrode 114 and thesource metal layer 122, the drain structure containing the drainelectrode 115 and the drain metal layer 123, the doped compoundsemiconductor layer 116, the dielectric layer 117, and the gatestructure containing the gate electrode 118 and the gate metal layer119. The substrate 110 may be a doped (such as doped with p-type orn-type dopants) or an undoped semiconductor substrate. For example, thesubstrate 110 may include an elemental semiconductor including Si or Ge;a compound semiconductors including GaAs, GaP, InP, InAs and/or InSb; analloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs,GaInP, and/or GaInAsP, or a combination thereof. In some embodiments,the substrate 110 may be a semiconductor-on-insulator substrate, such asa silicon-on-insulator substrate or a silicon germanium-on-insulator(SGOI) substrate. In other embodiments, the substrate 110 may be aceramic substrate, such as AlN substrate, SiC substrate, Al₂O₃ substrate(also referred to as a sapphire substrate), or another substrate. Insome embodiments, the substrate 110 may include a ceramic substrate anda pair of blocking layers disposed on the upper and lower surfaces ofthe ceramic substrate respectively. The material of the ceramicsubstrate may include a ceramic material, and the ceramic materialincludes an inorganic metal material. For example, the material of theceramic substrate may include SiC, AlN, sapphire, or another suitablematerial. The sapphire substrate may be alumina.

The channel layer 112 is on the substrate 110. In some embodiments, thematerial of the channel layer may include a binary compoundsemiconductor of group III-V, such as a nitride of group III. In someembodiments, the material of the channel layer may be GaN. In someembodiments, the channel layer may be doped with n-type or p-typedopants. The channel layer may be formed by an epitaxial growth process,such as MOCVD, hydride vapor phase epitaxy (HVPE), molecular beamepitaxy (MBE), a combination thereof, or the like. In some embodiments,the breakdown voltage of a HEMT is mostly determined by the thickness ofthe GaN channel layer. For example, the breakdown voltage of a HEMT maybe increased by about 100 volts as the thickness of the GaN channellayer is increased by 1 μm. During an epitaxial growth process forforming a GaN layer, a substrate with high thermal conductivity and highmechanical strength should be used to deposit GaN material thereon,otherwise the substrate may be warped or even cracked. Compared to theSi substrate, the AlN substrate has higher thermal conductivity andhigher mechanical strength such that a thicker GaN layer may be formedon the AlN substrate. For example, the thickness of the GaN layer formedon the surface of the Si substrate is between about 2 μm and about 4 μm,while the thickness of the GaN layer formed on the surface of the AlNsubstrate may be between about 5 μm and about 15 μm.

Since the crystal lattice and the coefficient of thermal expansion ofthe channel layer 112 may be different from those of the substrate 110,strains may occur at or near the interface between the channel layer 112and the substrate 110, resulting in defects such as cracks or warpage inthe channel layer 112. In some embodiments, the semiconductor structure200 may include the buffer layer 111 between the substrate 110 and thechannel layer 112, as shown in FIG. 2 . The strains of the channel layer112 subsequently formed over the buffer layer 111 may be mitigatedthrough the buffer layer 111 to prevent defects from forming in thechannel layer 112. The material of the buffer layer 111 may include AlN,GaN, Al_(x)Ga_(1-x)N (0<x<1), a combination thereof, or anothermaterial, and may be formed by an epitaxial growth process, such asMOCVD, HVPE, MBE, a combination thereof, or the like.

Although the buffer layer 111 shown in FIG. 2 is a single-layerstructure, the buffer layer 111 may be a multilayer structure (notshown). For example, the buffer layer 111 may include a superlatticebuffer layer and/or gradient buffer layer. The superlattice buffer layermay be disposed on the substrate 110 and the gradient buffer layer maybe disposed on the superlattice buffer layer, which may effectivelyprevent dislocations in the substrate 110 from entering the channelregion and further improve crystallization quality of other overlyingfilm(s) and/or layer(s). In addition, the superlattice buffer layer andthe gradient buffer layer may also be multilayer structures. Forexample, the superlattice buffer layer may include a plural sets ofalternating layers, and each set of the alternating layers may includeat least one AlN layer and at least one Al_(x)Ga_((1-x))N layer (0≤x<1,depending on the amount of Al) alternately arranged; and the gradientbuffer layer may include a plurality of Al_(y)Ga_((1-y))N layers (0≤y<1,depending on the amount of Al).

In some embodiments, a seed layer (not shown) may be formed between thesubstrate 110 and the buffer layer 111. The material of the seed layermay include AlN, Al₂O₃, AlGaN, SiC, Al, a combination thereof, or thelike. The seed layer may be a single-layer or a multilayer structure.The seed layer may be formed by a process the same as or similar to theepitaxial growth process described above. In some embodiments, thematerial of buffer layer 111 may be determined by the material of theseed layer and the inflow gas during the epitaxial growth process.

The barrier layer 113 is disposed on the channel layer 112. The materialof the barrier layer 113 may include a ternary compound semiconductor ofgroup III-V, such as a nitride of group III. For example, the materialof the barrier layer may include AlGaN, AlInN, or a combination thereof.In other embodiments, the barrier layer 113 may include GaN, AlN, GaAs,GaInP, AlGaAs, InP, InAlAs, InGaAs, other suitable material of groupIII-V, or a combination thereof. In some embodiments, the barrier layer113 may be doped with, for example, n-type or p-type dopants. Thebarrier layer may be formed by an epitaxial growth process, such asMOCVD, HVPE, MBE, a combination thereof, or a similar process. Accordingto some embodiments of the present disclosure, the material of thechannel layer 112 is different from that of the barrier layer 113, andthereby the interface between the channel layer 112 and the barrierlayer 113 is a heterojunction structure. The lattice mismatch betweenthe channel layer 112 and barrier layer 113 may result in stress thatleads to piezoelectric polarization effect. Besides, the ionicity of thebonding between the metal of group III (e.g. Al, Ga, or In) and nitrogenis relatively strong, resulting in spontaneous polarization. Due to thedifference in energy gap between the channel layer 112 and the barrierlayer 113, the piezoelectric polarization and spontaneous polarizationdescribed above, two-dimensional electron gas (2DEG) (not shown) isformed at the heterogeneous interface between the channel layer 112 andthe barrier layer 113. In some embodiments of the present disclosure,some semiconductor devices are HEMTs by using 2DEG as conductivecarriers.

Referring to FIG. 2 , the doped compound semiconductor layer 116 isdisposed on the barrier layer 113. The doped compound semiconductorlayer 116 has the side E₁ adjacent to the source structure, the side E₂adjacent to the drain structure, and the opening OP₁. The distancebetween the side E₁ and the side E₂ is D₁ and the width of the openingOP₁ is W₁. A portion of the barrier layer 113 is exposed through theopening OP₁. The doped compound semiconductor layer 116 may inhibit thegeneration of 2DEG under the gate electrode 118 to be formed thereon anda normally-off status for the semiconductor device is attained. Duringthe process, the doped compound semiconductor layer 116 may bedeactivated due to the environment (e.g. temperature or elements in theenvironment). In some embodiments of the present disclosure, the dopedcompound semiconductor layer 116 with the opening OP₁ may reduce thearea of the compound semiconductor layer 116 in semiconductor structure200 to lower the proportion of the compound semiconductor layer 116 indesign, thereby obviating the performance degradation of the device dueto the impact of environmental factors on the doped compoundsemiconductor layer 116 during the process. In some embodiments, thewidth W₁ of the opening the OP₁ is between ⅓ and ⅔ of the distance D₁between the side E₁ and the side E₂ such that the performancedegradation of the device described above may be obviated withoutsubstantially changing the function and properties of the doped compoundsemiconductor layer 116. In addition, since the area of the dopedcompound semiconductor layer 116 is reduced, the degradation ofcontrollability of the gate electrode 118 under the impact ofenvironmental factors acting on the doped compound semiconductor layer116 is obviated to improve the capability to drive current.

According to some embodiments of the present disclosure, the material ofthe doped compound semiconductor layer 116 may include GaN doped withp-type or n-type dopants. The thickness of the doped compoundsemiconductor layer 116 may be in a range from about 50 nm to about 150nm. The steps for forming the doped compound semiconductor layer 116 mayinclude depositing a doped compound semiconductor material layer on thebarrier 113 thorough an epitaxial growth process and forming a patternedmask layer on the doped compound semiconductor material layer,performing an etching process on the doped compound semiconductormaterial layer to remove portions of the doped compound semiconductormaterial layer uncovered by the patterned mask layer, and therebyforming the doped compound semiconductor layer 116 corresponding towhere the gate electrode 118 is to be formed. The patterned mask layeris then removed. The patterned mask layer may be hard mask orphotoresist. In some embodiments, the doped compound semiconductormaterial layer may be in-situ deposited in the same deposition chamberas the seed layer, the buffer layer 111, and the barrier layer 113. Inaddition, the doped compound semiconductor layer 116 may have arectangular cross-section as shown in FIG. 2 , or may have across-section of another shape, such as a trapezoidal shape.Furthermore, the upper surface of the doped compound semiconductor layer116 may not be flat.

In other embodiments, the doped compound semiconductor layer 116 mayinclude other semiconductors of group III-V dope with p-type dopants,such as AlGaN, AN, GaAs, AlGaAs, InP, InAlAs, or InGaAs. Further, thedoped compound semiconductor layer 116 may include other semiconductorsof group II-VI dope with p-type dopants, such as CdS, CdTe, or ZnS. Insome embodiments, the doped compound semiconductor layer 116 may bedoped with an element such as Li, Be, C, Na, Mg, Zn, Ca, Sr, Ba, Ra, Agand so on, and thereby the doped compound semiconductor layer 116 isp-type doped.

Still referring to FIG. 2 , the dielectric layer 117 is disposed on thebarrier layer 113 and the doped compound semiconductor layer 116, thegate electrode 118 is disposed on the doped compound semiconductor layer116 and embedded in the dielectric layer 117, and the gate metal layer119 is disposed on the dielectric layer 117 and may be used as a gatefield plate. As described above, the gate electrode 118 is disposed onthe doped compound semiconductor layer 116, and the generation of 2DEGunder the gate electrode 118 may be inhibited by the doped compoundsemiconductor layer 116 to attain a normally-off status for thesemiconductor device. The dielectric layer 117 may include a singlelayer or multiple layers of one or more dielectric materials, such assilicon oxide, silicon nitride, silicon oxynitride, tetraethoxysilane(TEOS), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG),low-k dielectric material, and/or another suitable dielectric material.The low-k dielectric material may include (but is not limited to)fluorinated silica glass (FSG), hydrogen silsesquioxane (HSQ),carbon-doped silicon oxide, amorphous fluorinated carbon, parylene,bis-benzocyclobutenes (BCB), or polyimide. The dielectric layer 117 maybe formed by, for example, spin-coating, chemical vapor deposition(CVD), atomic layer deposition (ALD), high-density plasma chemical vapordeposition (HDPCVD), other suitable methods, or a combination thereof.

The material of the gate electrode 118 may include a conductivematerial, such as metal, metal nitride, or semiconductor material. Forexample, the metal may include Au, Ni, Pt, Pd, Ir, Ti, Cr, W, Al, Cu,other suitable material, a combination thereof, or multiple layersthereof; the metal nitride may include MoN, WN, TiN, TaN, or anothersuitable material; the semiconductor material may includepolycrystalline silicon or polycrystalline germanium. The conductivematerial may be formed by a deposition process, such as CVD, ALD, orphysical vapor deposition (PVD) (e.g. sputtering or evaporation), andthen the conductive material is patterned to form the gate electrode118. In some embodiments, the gate metal layer 119 may be formed by asimilar method. The gate metal layer 119 may include a material the sameas or similar to the material of the gate electrode 118, and may beformed in the same process or in different processes. The material ofthe gate metal layer 119 may include NiSi, CoSi, TaC, TaSiN, TaCN, TiAl,TiAlN, metal oxide, metal alloy, other suitable conductive materials, ora combination thereof.

As shown in FIG. 2 , the source structure and the drain structure aredisposed on opposing sides of the barrier layer 113. The sourcestructure may include the source electrode 114 and the source metallayer 122 disposed on the source electrode 114, and the drain structuremay include the drain electrode 115 and the drain metal layer 123disposed on the drain electrode 115. In some embodiments, the method forforming the source electrode 114 and the drain electrode 115 includesperforming a patterning process on the barrier layer to form a pair ofopenings (or more openings) in the barrier layer, filling a conductivematerial into the openings, performing a planarization process (such aschemical mechanical polishing) or an etch-back process to remove excessmaterial outside the openings, and thereby forming the source electrode114 and drain electrode 115. In other embodiments, the source electrode114 and drain electrode 115 may be formed through a similar process asdescribed above after the formation of the dielectric layer 117. Thematerial and method of forming the conductive material is similar tothose of the conductive material of the gate electrode 118. In someembodiments, the source metal layer 122 or the drain metal layer 123 maybe formed by a similar method in the subsequent process. According tosome embodiments of the present disclosure, the method of forming thegate electrode 118 may be similar to the method of forming the sourceelectrode 114 and the drain electrode 115.

In some embodiments, the source metal layer 122 may be directly on andin direct contact with the source electrode 114, or may be electricallyconnected to the source electrode 114 through a contact. Similarly, thedrain metal layer 123 may be directly on and in direct contact with thedrain electrode 115, or may be electrically connected to the drainelectrode 115 through a contact. For example, the source electrode 114of the source structure is embedded in the dielectric layer 117, thesource metal layer 122 of the source structure may be disposed on thedielectric layer 117, and the source electrode 114 and the source metallayer 122 are electrically connected through a source contact embeddedin the dielectric layer 117. The potential of the source metal layer 122electrically connected to the source electrode 114 is different from thepotential of the gate metal layer 119 electrically connected to the gateelectrode 118. In these embodiments, the source metal layer 122 extendsalong a direction from the source structure to the drain structure andfunctions as a source field plate, thereby reducing the intensity ofelectric field. In other embodiments, the source electrode 114 and thedrain electrode 115 may penetrate through the barrier layer 113 andcontact with the channel layer 112. The source metal layer 122 and thedrain metal layer 123 may include the same or a similar material as thesource electrode 114 and the drain electrode 115, and may be formed inthe same process or in different processes. In some embodiments, thematerial of the source metal layer 122 and the drain metal layer 123 mayinclude NiSi, CoSi, TaC, TaSiN, TaCN, TiAl, TiAlN, metal oxide, metalalloy, other suitable conductive materials, or a combination thereof.

As described above, the doped compound semiconductor layer 116 may bedeactivated due to influence of the environment (e.g. temperature orelements in the environment) during the process. The deactivation maydegrade the controllability of the gate electrode 118 and thereby reducethe capability to drive current. Therefore, to reduce the proportion ofthe doped compound semiconductor layer 116 in device design, someembodiments of the present disclosure provide a doped compoundsemiconductor layer 116 with the opening OP₁, which may reduce the areaof the compound semiconductor layer 116 in the semiconductor structure200. This may obviate the lower controllability of the gate electrode118 under the impact of environmental factors acting on the dopedcompound semiconductor layer 116 and improve the capability to drivecurrent and electrical uniformity. For example, in the same condition(for example, the same voltage), the doped compound semiconductor layer116 with the opening OP₁ may increase the driving current of the deviceby about 25% or more. According to some embodiments, the width W₁ isbetween ⅓ and ⅔ of the distance D₁ between the side E₁ and the side E₂such that the degradation of controllability of the gate electrode 118described above may be obviated without substantially changing thefunction and properties of the doped compound semiconductor layer 116,and the capability to drive current may be improved.

In some embodiments, the semiconductor structure 200 may further includea protection layer and a liner layer (not shown). The protection layermay be disposed on sidewalls and partial upper surface of the dopedcompound semiconductor layer 116, and on partial upper surface of thebarrier layer 113. In some embodiments, the protection layer may repairthe lattice defects on the sidewalls of the doped compound semiconductorlayer 116 caused by the etch process and thereby reduce the gate leakagecurrent of the resulting semiconductor device. Furthermore, theprotection layer formed on partial upper surface of the barrier layer113 may prevent the barrier layer 113 from oxidation to enhance theperformance of the resulting semiconductor device. Depending on therequirement of process or device design, the thickness of the protectionlayer may be in a range from about 0.5 nm to about 500 nm. The materialof the protection layer may include an insulation material or adielectric material, such as SiO₂, SiN, SiON, Al₂O₃, AlN, MgO, Mg₃N₂,ZnO, TiO₂, a combination thereof, or similar materials.

In some embodiments, the material of the protection layer may includenitride, such as silicon nitride or aluminum nitride, which maypreferably repair the lattice defects on the sidewalls of the dopedcompound semiconductor layer 116. In some embodiments, a layer of theabove material may be formed on the substrate 110 by CVD, plasmaenhanced CVD (PECVD), ALD, PVD (such as sputtering), or the like, andthe material layer may be patterned to form the protection layer. Inother embodiments, the protection layer on the upper surface of thedoped compound semiconductor layer 116 may be entirely removed by apatterning process such that the protection layer is on sidewalls of thedoped compound semiconductor layer 116 and on the upper surface of thebarrier layer 113.

In some embodiments, the liner layer may be disposed on bottoms andpartial sidewalls of the source electrode 114 and the drain electrode115, and on partial upper surface of the barrier layer 113. In someembodiments, the liner layer may be helpful to generate more 2DEG on theheterogeneous interfaces of the source electrode 114 and the drainelectrode 115. This may reduce the contact resistance (R_(contact))between the source electrode 114/drain electrode 115 and the channellayer 112 and further reduce the on-state resistance of thesemiconductor structure. Furthermore, the liner layer formed on theupper surface of the barrier layer 113 may prevent the barrier layer 113from oxidation and thereby improve the performance of the device.

In some embodiments, the material of the liner layer may include abinary compound semiconductor with hexagonal crystal structure, such asAlN, ZnO, InN, a combination thereof, or a similar material, and may beformed by ALD or an epitaxial growth process, such as MOCVD. In anembodiment where the liner layer is formed by MOCVD, since MOCVD is aselective area growth (SAG) process, the liner layer is formed onportions of the barrier layer 113 uncovered by the protection layer andinterfaces with the protection layer without being formed on theprotection layer. In another embodiment where the liner layer is formedby ALD, the liner layer is formed on portions of the barrier layer 113uncovered by the protection layer and extends onto the protection layer.Besides, in some other embodiments, the material of the liner layer mayinclude graphene with hexagonal crystal structure, and the liner layermay be formed by CVD or ALD. In some embodiments, the material of theliner layer may be the same as the material of the protection layer, forexample, both are AlN. In other embodiments, the material of the linerlayer is different from the material of the protection layer, forexample, the liner layer is AlN and the protection layer is SiN.

FIG. 3 illustrates a top view of a portion of a semiconductor structureand FIG. 4 illustrates a cross-sectional view of the semiconductorstructure along the A-A′ line in FIG. 3 , in accordance with someembodiments of the present disclosure. The semiconductor structure 400is similar to the semiconductor structure 200 except the doped compoundsemiconductor layer 316 of the semiconductor structure 400 has twoopenings. For simplicity, like features in FIG. 4 and FIG. 2 aredesignated with like reference numerals and the description is notrepeated. As shown in FIG. 4 , the doped compound semiconductor layer316 has the side E₃ adjacent to the source structure, the side E₄adjacent to the drain structure, and the openings OP_(3a) and OP_(3b).The distance between the side E₃ and the side E₄ is D₃, the width of theopening OP_(3a) is W_(3a), and the width of the opening OP_(3b) isW_(3b). Portions of the barrier layer 113 are exposed through theopenings. The semiconductor structure 400 includes the doped compoundsemiconductor layer 316 with the openings OP_(3a) and OP_(3b), which mayreduce the area of the compound semiconductor layer 316 in thesemiconductor structure 400. As described above, the degradation ofcontrollability of the gate electrode 118 due under the impact ofenvironmental factors acting on the doped compound semiconductor layer316 is obviated and the capability to drive current is improved.

According to some embodiments, the sum of the widths W_(3a) and W_(3b)of the respective openings OP_(3a) and OP_(3b) is between ⅓ and ⅔ of thedistance D₃ between the side E₃ and the side E₄ such that thedegradation of controllability of the gate electrode 118 described abovemay be obviated without substantially changing the function andproperties of the doped compound semiconductor layer 116, therebyimproving the capability to drive current. It should be noted that thenumber of openings shown in FIG. 4 is only an example, and the number ofopenings of the compound semiconductor layer in the embodiment of thepresent disclosure may be more than two. In some embodiments where thecompound semiconductor layer has more than two openings, the sum of thewidth of the opening is between ⅓ and ⅔ of the distance between a sideof the doped compound semiconductor layer adjacent to the sourcestructure and another side of the doped compound semiconductor layeradjacent to the drain structure. As described above, the controllabilityof the gate electrode and the capability to drive current may beimproved without substantially changing the function and properties ofthe doped compound semiconductor layer. In some embodiments, thesemiconductor structure 400 may also include, for example, a protectionlayer and/or a liner layer to reduce the leakage current of theresulting semiconductor device and/or the on-state resistance of thesemiconductor structure 400, and prevent the surface of the barrierlayer 113 from oxidation. This may improve the performance of theresulting device. Further, as shown in FIG. 3 , the doped compoundsemiconductor layer 316 has a rectangular opening the OP_(3a) and anoval opening the OP_(3b). Specifically, in the A-A′ direction from thesource electrode 114 to the drain electrode 115 (or from the sourcestructure to the drain structure), the doped compound semiconductorlayer 316 has two openings OP_(3a) and OP_(3b), and the openings have amaximum sum of widths (W_(3a)+W_(3b)) in the A-A′ direction. In someembodiments, in order to maintain the function and properties of thedoped compound semiconductor layer 316, the maximum sum of the widths(W_(3a)+W_(3b)) of the openings OP_(3a) and OP_(3b) in the A-A′direction is between ⅓ and ⅔ of the maximum width D₃ of the dopedcompound semiconductor layer 316 in the A-A′ direction, and the minimumdistance between the openings OP_(3a) and OP_(3b) in the A-A′ directionis the average of the widths of the openings OP_(3a) and OP_(3b) in theA-A′ direction, that is (W_(3a)+W_(3b))/2. As described above, the dopedcompound semiconductor layer 316 with openings may improve the gatecontrollability of the device and enhance the capability to drivecurrent.

FIGS. 5-11 illustrate top views of portions of semiconductor structures,in accordance with some varied embodiments of the present disclosure.Referring to FIG. 5 , the semiconductor structure includes the sourceelectrode 114, the drain electrode 115, and the doped compoundsemiconductor layer 516. As shown in the top view, the doped compoundsemiconductor layer 516 has the rectangular opening OP_(5a), the ovalopening OP_(5b), and the triangle opening OP_(5c). Specifically, in theA-A′ direction from the source electrode 114 to the drain electrode 115(or from the source structure to the drain structure), the dopedcompound semiconductor layer 516 has three openings OP_(5a), OP_(5b),and OP_(5c), and the openings have a maximum sum of widths(W_(5a)+W_(5b)+W_(5c)) in the A-A′ direction. In some embodiments, formaintaining the function and properties of the doped compoundsemiconductor layer 516, the maximum sum of widths (W_(5a)+W_(5b) W₅O inthe A-A′ direction is between ⅓ and ⅔ of the maximum width D₅ of thedoped compound semiconductor layer 516 in the A-A′ direction, and theminimum distance between adjacent ones of the three openings OP_(5a),OP_(5b), and OP_(5c) in the A-A′ direction is the average of the widthsof the openings OP_(5a), OP_(5b), and OP_(5c) in the A-A′ direction,which is (W_(5a)+W_(5b) W_(5c))/3. The doped compound semiconductorlayer 516 with openings may improve the gate controllability of thedevice and enhance the capability to drive current.

Referring to FIG. 6 , the semiconductor structure includes the sourceelectrode 114, the drain electrode 115, and the doped compoundsemiconductor layer 616. As shown in the top view, the doped compoundsemiconductor layer 616 has the rectangular openings OP_(6a) andOP_(6b). In the A-A′ direction from the source electrode 114 to thedrain electrode 115 (or from the source structure to the drainstructure), at least a portion of the doped compound semiconductor layer616 is discontinuous. The opening OP_(6a) has a width W_(6a) in adirection parallel to the A-A′ direction, and in the A-A′ direction, theopening OP_(6b) has a width W_(6b) greater than the width W_(6a). Insome embodiments, for maintaining the function and properties of thedoped compound semiconductor layer 616, the greater width W_(6b) of theopening OP_(6b) is between ⅓ and ⅔ of the maximum width D₆ of the dopedcompound semiconductor layer 616 in the A-A′ direction, and the minimumvalue of the spacing S₁ between the openings OP_(6a) and OP_(6b) in theB-B′ direction perpendicular to the A-A′ direction is ½ of the maximumwidth D₆ of the doped compound semiconductor layer 616 in the A-A′direction. As discussed above, the doped compound semiconductor layer616 with openings may improve the gate controllability of the device andenhance the capability to drive current.

Referring to FIG. 7 , the semiconductor structure includes the sourceelectrode 114, the drain electrode 115, and the doped compoundsemiconductor layer 716. As shown in the top view, the doped compoundsemiconductor layer 716 has the rectangular opening OP_(7a), thetrapezoidal opening OP_(7b), and the circle opening OP_(7c). In the A-A′direction from the source electrode 114 to the drain electrode 115 (orfrom the source structure to the drain structure), at least a portion ofthe doped compound semiconductor layer 716 is discontinuous. The openingOP_(7a) has a width W_(7a) in a direction parallel to the A-A′direction, the wider bottom of the opening OP_(7b) has a width W_(7b) inthe A-A′ direction, the opening OP_(7c) has a width W_(7c) in anotherdirection parallel to the A-A′ direction, and the width W_(7b) isgreater than the widths W_(7a) and W_(7b). In some embodiments, formaintaining the function and properties of the doped compoundsemiconductor layer 716, the greater width W_(7b) is between ⅓ and ⅔ ofthe maximum width D₇ of the doped compound semiconductor layer 716 inthe A-A′ direction. In addition, the minimum value of the spacing S₂between the openings OP_(7a) and OP_(7b) and the minimum value of thespacing S₃ between the openings OP_(7b) and OP_(7c) in the B-B′direction perpendicular to the A-A′ direction are ½ of the maximum widthD₇ of the doped compound semiconductor layer 716 in the A-A′ direction.As discussed above, the doped compound semiconductor layer 716 withopenings may improve the gate controllability of the device and enhancethe capability to drive current.

The shapes of the openings of the doped compound semiconductor layerdescribed above are only examples and not limited. The shapes of theopenings may include a rectangle, diamond, trapezoid, circle, oval,triangle, or a combination thereof. Besides, the embodiments of thepresent disclosure may also be applied to openings with irregularshapes.

In addition to the openings, the doped compound semiconductor layer mayhave at least one notch (or namely opening) on at least one of the twosides E₅ and E₆ parallel to the A-A′ direction, as shown in FIG. 8 . Thesemiconductor structure includes the source electrode 114, the drainelectrode 115, and the doped compound semiconductor layer 816 having arectangular notch N₈. In the A-A′ direction from the source electrode114 to the drain electrode 115 (or from the source structure to thedrain structure), at least a portion of the doped compound semiconductorlayer 816 is discontinuous. The extension direction of the notch N₈ isperpendicular to the A-A′ direction and the notch N₈ has a maximum widthW₈ in the A-A′ direction. In some embodiments, for maintaining thefunction and properties of the doped compound semiconductor layer 816,the maximum width W₈ of the notch N₈ in the A-A′ direction is between ⅓and ⅔ of the maximum width D₈ of the doped compound semiconductor layer816 in the A-A′ direction. Through the doped compound semiconductorlayer 816 with the notch, the area of the doped compound semiconductorlayer in the semiconductor structure may be reduced. Therefore, thedegradation of controllability of the gate electrode due to the impactof environmental factors on the doped compound semiconductor layer isobviated to improve the capability to drive current.

The doped compound semiconductor layer may have more than one notch onat least one of the two sides E₇ and E₈ parallel to the A-A′ directionand form an M-shape or a comb shape, as shown in FIG. 9 . Thesemiconductor structure includes the source electrode 114, the drainelectrode 115, and the doped compound semiconductor layer 916 having anotch N_(9a) with a shape of partial oval and a notch N_(9b) with ashape of triangle. In the A-A′ direction from the source electrode 114to the drain electrode 115 (or from the source structure to the drainstructure), at least a portion of the doped compound semiconductor layer916 is discontinuous. The extension directions of the two notches areperpendicular to the A-A′ direction and the two notches have a maximumsum of widths (W_(9a)+W_(9b)) in the A-A′ direction. In someembodiments, in order to maintain the function and properties of thedoped compound semiconductor layer 916, the maximum sum of widths(W_(9a)+W_(9b)) of the notches in the A-A′ direction is between ⅓ and ⅔of the maximum width D₉ of the doped compound semiconductor layer 916 inthe A-A′ direction, and the minimum distance between the notches N_(9a)and N_(9b) in the A-A′ direction is the average of the widths of thenotches N_(9a) and N_(9b) in the A-A′ direction, that is(W_(9a)+W_(9b))/2. The doped compound semiconductor layer 916 withnotches may improve the gate controllability of the device and enhancethe capability to drive current.

Referring to FIG. 10 , the semiconductor structure includes the sourceelectrode 114, the drain electrode 115, and the doped compoundsemiconductor layer 1016. The doped compound semiconductor layer 1016 isU-shaped as shown in top view. In the A-A′ direction from the sourceelectrode 114 to the drain electrode 115 (or from the source structureto the drain structure), at least a portion of the doped compoundsemiconductor layer 1016 is discontinuous. The extension direction ofthe notch of the doped compound semiconductor layer 1016 isperpendicular to the A-A′ direction and the notch has a maximum widthW₁₀ in the A-A′ direction. In some embodiments, in order to maintain thefunction and properties of the doped compound semiconductor layer 1016,the maximum width W₁₀ of the notch in the A-A′ direction is between ⅓and ⅔ of the maximum width D₁₀ of the doped compound semiconductor layer1016 in the A-A′ direction. The doped compound semiconductor layer 1016with the notch may improve the gate controllability of the device andenhance the capability to drive current as described above.

The shapes of the notches of the doped compound semiconductor layerdescribed above are only examples and not limited. The shapes of thenotches may include a rectangle, trapezoid, partial circle, partialoval, triangle, or a combination thereof. The shapes of the dopedcompound semiconductor layer with notches are not limited, which mayinclude M-shape, U-shape, comb, or a combination thereof.

Referring to FIG. 11 , the semiconductor structure includes the sourceelectrode 114, the drain electrode 115, and the doped compoundsemiconductor layer 1116 including a plurality of discrete dopedcompound semiconductor islands. The doped compound semiconductor islandsinclude the rectangular doped compound semiconductor island 1116 a, andthe oval doped compound semiconductor islands 1116 b and 1116 c. In theA-A′ direction from the source electrode 114 to the drain electrode 115(or from the source structure to the drain structure), the dopedcompound semiconductor layer 1116 is discontinuous, and the dopedcompound semiconductor islands 1116 a, 1116 b, and 1116 c have a maximumsum of widths (W_(11a)+W_(11b)+W_(11c)) in the A-A′ direction. In someembodiments, in order to maintain the function and properties of thedoped compound semiconductor layer 1116, the maximum sum of widths(W_(11a)+W_(11b)+W_(11c)) of the doped compound semiconductor islands1116 a, 1116 b, and 1116 c in the A-A′ direction is between ½ and 2times the sum of the spacings (S₄+S₅) between the adjacent dopedcompound semiconductor islands in the A-A′ direction. The doped compoundsemiconductor layer 1116 including a plurality of discrete dopedcompound semiconductor islands may also reduce the area of the dopedcompound semiconductor layer in the semiconductor structure, and therebyenhance the capability to drive current. The shapes of doped compoundsemiconductor islands are only examples and not limited, and may includea rectangle, trapezoid, circle, oval, triangle, or a combinationthereof.

FIG. 12 illustrates a comparison between a comparative example and theexample shown in FIG. 6 . The Y-axis is the intensity of the electricfield between the channel layer and the barrier layer near the surfaceof the semiconductor structure, and the X-axis is the correspondingposition of the intensity of the electric field in the horizontaldirection of the semiconductor structure. The origin of the X-axis is aposition near the source in the semiconductor structure, and thecorresponding position moves away from the position near the sourcetoward the drain as the coordinate on the X-axis increases. Thecomparative example represents a doped compound semiconductor layerwithout any opening or notch, while the example in FIG. 6 effectivelyreduces the electric field near the surface by reducing the area of thedoped compound semiconductor layer.

The embodiments of the present disclosure provides a doped compoundsemiconductor layer with one or more openings or notches, or a dopedcompound semiconductor layer with a discontinuous structure such thatthe area of the compound semiconductor layer in the semiconductorstructure may be reduced. Therefore, the degradation of controllabilityof the gate electrode under the impact of environmental factors actingon the doped compound semiconductor layer during processes is obviatedto improve the capability to drive current and electrical uniformity,and to further improve device performance. In addition, thesemiconductor structure with a reduced area of the doped compoundsemiconductor layer may also reduce the electric field near the surfaceto achieve REduced Surface Field (RESURF). The above description is onlyone of the purposes of the present disclosure and it is not intended tolimit the scope of present disclosure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: asubstrate; a channel layer on the substrate; a barrier layer on thechannel layer; a source structure and a drain structure on oppositesides of the barrier layer, and a first direction from the sourcestructure to the drain structure; a doped compound semiconductor layeron the barrier layer, wherein the doped compound semiconductor layer hasa first side adjacent to the source structure, a second side adjacent tothe drain structure, and the doped compound semiconductor layer has atleast one opening exposing a portion of the barrier layer, wherein amaterial of the doped compound semiconductor layer comprises GaN; adielectric layer on the doped compound semiconductor layer and thebarrier layer; and a gate structure on the doped compound semiconductorlayer and electrically connected to the doped compound semiconductorlayer.
 2. The semiconductor structure as claimed in claim 1, wherein awidth of the at least one opening is between ⅓ and ⅔ of a distancebetween the first side and the second side of the doped compoundsemiconductor layer.
 3. The semiconductor structure as claimed in claim1, wherein the at least one opening comprises a plurality of openings,and a sum of widths of the openings is between ⅓ and ⅔ of a distancebetween the first side and the second side of the doped compoundsemiconductor layer.
 4. The semiconductor structure as claimed in claim1, further comprising a buffer layer between the substrate and thechannel layer.
 5. The semiconductor structure as claimed in claim 1,wherein the at least one opening comprises a plurality of openings, aminimum distance between adjacent ones of the openings in a seconddirection perpendicular to the first direction is ½ of the maximum widthof the doped compound semiconductor layer in the first direction.
 6. Thesemiconductor structure as claimed in claim 1, wherein a shape of the atleast one opening comprises a rectangle, diamond, trapezoid, circle,oval, triangle, or a combination thereof.
 7. The semiconductor structureas claimed in claim 1, wherein the at least one opening is disposed atone side of the doped compound semiconductor layer and the side of thedoped compound semiconductor layer is parallel to the first direction.8. The semiconductor structure as claimed in claim 7, wherein anextension direction of the at least one opening is perpendicular to thefirst direction.
 9. The semiconductor structure as claimed in claim 7,wherein a shape of the doped compound semiconductor layer comprises anM-shape, U-shape, comb, or a combination thereof.
 10. The semiconductorstructure as claimed in claim 1, wherein the doped compoundsemiconductor layer comprises a plurality of discrete doped compoundsemiconductor islands.
 11. The semiconductor structure as claimed inclaim 10, wherein the doped compound semiconductor islands have amaximum sum of widths in the first direction, and the maximum sum isbetween ½ and 2 times a sum of the distances between adjacent dopedcompound semiconductor islands in the first direction.
 12. Thesemiconductor structure as claimed in claim 11, wherein a shape of thediscrete doped compound semiconductor islands comprises a rectangle,trapezoid, circle, oval, triangle, or a combination thereof.